Convert the `AdderToolbar` component to tailwind utility-first styles.

This component has some styling complexity that requires the continued
use of some external styles for now:

* It serves as the root element in a shadow DOM and has a couple of
  reset rules not serviceable by tailwind utility classes
* The hover behavior of the adder toolbar buttons is too complex to
  implement with utility styles alone.

Visual differences before/after are negligible. The adder is 1px taller
due to using standardized spacing.